def is_valid(isbn):
    '''Verify if an input ISBN string is valid.'''
    digits = []
    for i in list(isbn):
        if i.isdigit():
            digits.append(int(i))
        elif i == 'X':
            digits.append(10)
            break
        elif i != '-':
            return False
    if len(digits) != 10:
        return False
    isbn_sum = 0
    for i in range(10):
        isbn_sum += digits[i] * (10-i)
    return isbn_sum % 11 == 0